Trends in the M³⁺/M²⁺ Standard Electrode Potentials

What the M³⁺/M²⁺ potential measures

The standard electrode potential for the M³⁺/M²⁺ couple gauges how readily a metal's tripositive ion is reduced to its dipositive ion — equivalently, it measures the relative stability of the +3 state against the +2 state for that element. A low value (strongly negative, or only weakly positive) signals that M³⁺ resists reduction, so the +3 state is the comparatively stable one for that metal. A high, strongly positive value signals the reverse: M³⁺ is readily reduced, the +2 state is favoured, and M³⁺ itself behaves as an oxidising agent in aqueous solution.

Trends set by especially stable d-electron configurations

Examined across the first-row series, the M³⁺/M²⁺ values are far from a smooth progression. The irregularities line up closely with the same handful of unusually stable d-electron arrangements that recur throughout the chemistry of this series — the empty ($d^0$), half-filled ($d^5$) and completely filled ($d^{10}$) configurations.

Scandium shows an unusually low value, consistent with the exceptional stability of Sc³⁺, which has already attained a noble-gas-like $d^0$ configuration — there is little to be gained by reducing it further to Sc²⁺.

Zinc shows the highest value in the series. Zn²⁺ already carries the fully filled and highly stable $3d^{10}$ configuration, so oxidising it further to Zn³⁺ would mean breaking into that stable sub-shell — an energetically costly step. Correspondingly, the reverse process, reduction of Zn³⁺ back to Zn²⁺, is strongly favoured.

Manganese shows a comparatively high value because Mn²⁺, with its half-filled $3d^5$ configuration, is an especially stable ion. Mn³⁺ is therefore readily reduced to it, which makes Mn³⁺ a comparatively strong oxidant in solution.

Iron shows a comparatively low value for the opposite reason: here it is Fe³⁺ (also $3d^5$) that carries the stable half-filled configuration, so the +3 state is the favoured one and Fe³⁺ resists reduction.

Vanadium's comparatively low value is likewise tied to a stable configuration on the +2 side — V²⁺ corresponds to a half-filled $t_{2g}$ level, giving it extra stability.

The general picture …